Viewed against a background of the long-standing enmity between Iran and the West, the nuclear deal signed with Tehran is a stunning achievement. President Barack Obama has been seeking meaningful negotiations with Iran since coming to power but the bombast and confrontation of his counterpart, Mahmoud Ahmadinejad, left little room for any form of agreement. But matters were transformed five months ago when Hassan Rouhani, a moderate cleric, swept to victory in the Iranian presidential election. Since then, things have moved with remarkable swiftness, culminating in the deal signed in Geneva after five days of negotiation between Iran and the Security Council's permanent members and Germany.

It is impossible to overstate the impact of the new leadership and a new attitude in Tehran. These have opened the door to diplomacy. The presidential election had expressed what Iranians felt about the maintenance of a siege mentality, whatever their country's grievances with the West. It had also said much about the hardship caused by decades of economic sanctions, notably through the loss of oil sales.

The significance of Dr Rouhani's election was clearly not lost on the country's Supreme Leader, Ayatollah Ali Khamenei, who backed the negotiations with the West on the understanding that a "red line" guaranteeing Iran's right to enrich uranium for peaceful uses was not crossed.

Given animosity and mistrust dating, this time, from the 1979 Islamic Revolution, there was never any prospect that Tehran would kowtow to the West. What has been agreed, however, benefits both parties and represents a major step forward. Iran will curb many of its nuclear activities for six months in exchange for modest and gradual relief from sanctions. Its ability to enrich uranium will be frozen at a maximum 5 per cent, well below the threshhold for weapons-grade material. Most importantly, international monitors will oversee Iran's compliance, effectively ruling out the chance to develop nuclear weaponry without being detected.

The six-month period provides time to negotiate a more sweeping deal. Pessimists doubt this will happen. They note Iran signed interim nuclear pacts between 2003 and 2005, all of which fell apart when Tehran violated their terms. Ongoing criticism will also come from Israel, which described the agreement as a "historic mistake" that made the world a more dangerous place. The country's Prime Minister, Benjamin Netanyahu, declared that Israel "has the right and the duty to defend itself by itself".

Such rhetoric, if unwelcome, was hardly surprising. It was necessary if Israel was to show that it still believed it had the right to launch a military strike against Iranian nuclear facilities. But to all intents, the Geneva pact rules out such a raid. Israel would find itself isolated if it took action that destabilised the region just as matters seemed to be improving after decades of turmoil and 10 years of trying to get a deal with Iran.

It is, of course, premature to assume that a broad agreement removing any threat from Iran's nuclear programme will be reached in six months. There are still questions about the degree of support Dr Rouhani will receive from Ayatollah Khamenei. As a former nuclear negotiator, himself, the President is aware Iranian hard-liners will seek to undermine the outcome of negotiations. But there is a definite sense Iran is ready to seek a rapprochement, even while reserving the right to nuclear power for peaceful uses. If that breakthrough were achieved, there may also be room for other accommodations, not least over Syria, where Iran has been a strong supporter of President Bashar al-Assad. At the least, the Middle East would suddenly be a much more stable region.

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